1. Field of the Invention
An apparatus consistent with the present invention relates to a disk drive and, more particularly, to an actuator having an improved dynamic characteristic and a disk drive having the same.
2. Description of the Related Art
Hard disk drives (HDDs), which are data storage devices used for computers, use read/write heads to reproduce data from a disk or record data to a disk. In the HDD, the head performs its functions while being moved by an actuator to a desired position while maintaining a predetermined height from a recording surface of a rotating disk.
The storage capacity of the disk drive has continuously increased. The storage capacity of the disk drive is proportional to a surface recording density. The surface recording density is determined by the multiplication of a linear recording density represented by BPI (bits per inch) and a track density represented by TPI (tracks per inch). The increase of BPI is affected solely by the development of a magnetic recording technology while the increase of TPI is greatly dependent on the improvement of a dynamic characteristic of the actuator.
FIG. 1 is a perspective view illustrating the structure of a conventional disk drive. FIG. 2A is a plan view illustrating the conventional actuator shown in FIG. 1. FIG. 2B is a vertical sectional view taken along line A-A′ of FIG. 2A, showing the arrangement of a VCM coil and a magnet of FIG. 1.
Referring to FIGS. 1, 2A, and 2B, the conventional disk drive includes a disk 10 to store data, a spindle motor 20 to rotate the disk 10, and an actuator 30 to move a read/write head 34, which records and reproduces the data, to a desired position on the disk 10. The actuator 30 includes a swing arm 32 that is rotatably coupled to an actuator pivot 31, a suspension 33, which is installed at a leading end portion of the swing arm 32 to support the head 34 to be elastically biased toward a surface of the disk 10, and a voice coil motor (VCM) to rotate the swing arm 32. The voice coil motor includes a VCM coil 37 coupled to a coil support portion 36 provided at a rear end portion of the swing arm 32 and magnets 38 arranged above and under the VCM coil 37 to face the VCM coil 37. Each of the magnets 38, as shown in FIG. 2B, is magnetized to have two polarities, that is, an N pole and an S pole. The magnets 38 are typically supported by being attached to yokes 39.
The voice coil motor having the above structure rotates the swing arm 32 in a direction according to Fleming's left hand rule due to an interaction between a current applied to the VCM coil 37 and a magnetic field formed by the magnets 38. That is, when the disk drive is turned on and the disk 10 starts to rotate at a particular angular velocity Ω, the voice coil motor rotates the swing arm 32 in a predetermined direction, for example, counterclockwise, to move the head 34 above a recording surface of the disk 10. The head 34 is lifted to a predetermined height from a surface of the disk 10 by a lift force generated by the rotating disk 10. In this state, the head 34 follows a particular track 12 of the disk 10 to record or reproduce data with respect to the recording surface of the disk 10.
When the disk drive is turned off and the disk 10 stops rotating, the voice coil motor rotates the swing arm 32 in the opposite direction, for example, clockwise. Accordingly, the head 34 exits out of the recording surface of the disk 10.
In the above disk drive, the operation of actuator 30 is divided into a seeking operation to seek the desired particular track 12 out of a plurality of tracks on the disk 10 and a tracking operation to follow the particular track 12. Such operations are described below in detail.
As shown in FIG. 2A, when current flows through the VCM coil 37 in a particular direction, a force F acts on the VCM coil 37 in a direction following Fleming's left hand rule due to the interaction between the current and the magnetic field generated by the magnets 38. The force F acts on the VCM coil 37 at a right angle. Accordingly, the actuator 30 rotates around the actuator pivot 31 so that the head 34, provided at the leading end portion of the actuator 30, is moved.
As described above, the storage capacity of a disk drive increases gradually and accordingly the number of tracks on the disk 10 increases. When the number of tracks increases, that is, TPI increases, the width of each track decreases. Thus, in order for the head 34 to follow a track having a very narrow width without generating an error signal, that is, a position error signal (PES), the actuator 30, which moves the head 34, needs to be controlled more precisely.
However, the conventional actuator 30 has a variety of resonance modes, in which an in-plane resonance mode is known to exert the greatest effect on the tracking operation of the actuator 30.
FIG. 3 shows the shape of an in-plane resonance mode of the conventional actuator 30 of FIG. 2A. As shown in FIG. 3, the in-plane resonance mode in the conventional actuator 30 has a shape of the swing arm 32 and the coil support portion 36 being twisted at the same phase around the actuator pivot 31. Such in-plane resonance mode is typically referred to as a butterfly mode.
Referring back to FIG. 2A, the force F acting on the VCM coil 37 of the actuator 30 is divided into a component force in a direction X parallel to the direction of the track and a component force in a direction Y perpendicular to the direction of the track. Among them, the component force in the direction Y works as an exiting force to excite the butterfly mode.
FIG. 4 is a graph showing the in-plane response of the head in the tracking operation of the conventional actuator shown in FIG. 2A. Referring to FIG. 4, a peak P having a great magnitude appears at a frequency of about 4800 Hz and the peak P is due to the in-plane resonance mode, that is, the butterfly mode.
As described above, the in-plane resonance mode, that is, the butterfly mode, of the actuator 30 is excited by the component force in the direction Y acting on the VCM coil 37 so that the peak P having a great magnitude is generated. Accordingly, in the tracking operation of the actuator 30, the precise track following of the head 34 becomes difficult as the width of the track is decreased. Thus, the PES increases as the width of the track decreases. Also, as a bandwidth for servo control is restricted by the frequency and magnitude of the peak P, a response to a high frequency becomes defective so that performance of the disk drive is deteriorated.
FIGS. 5A and 5B are views illustrating an actuator having a dual VCM coil to solve the above problem. FIG. 5A shows the flow of current during a seeking operation while FIG. 5B shows the flow of current during a track following operation. The actuator shown in FIGS. 5A and 5B is disclosed in U.S. Pat. No. 6,104,581.
First, referring to FIG. 5A, a dual VCM coil having an outer coil 92 and an inner coil 94 is installed at a rear end portion of an actuator 90. In the actuator 90, when a seeking operation to search for a desired particular track is performed, the direction of current Io flowing through the outer coil 92 and the direction of current Ii flowing through the inner coil 94 are made identical. Then, since forces Fo and Fi act in the same direction on the outer coil 92 and the inner coil 94, respectively, the actuator 90 performs the seek operation at a fast speed.
Referring to FIG. 5B, when a tracking operation is performed, the direction of the current Io flowing through the outer coil 92 and the direction of the current Ii flowing through the inner coil 94 are made opposite. Because the direction of the force Fo acting on the outer coil 92 and the direction of the force Fi acting on the inner coil 94 are opposite, the component forces in the direction Y of the two forces Fo and Fi are offset so that the sum of the two component forces in the direction Y acting on the actuator 90 is minimized.
Although the in-plane butterfly mode exists at the actuator 90, the excitation by the component force in the direction Y is minimized when the head follows the track in the above structure. Thus, the height of the peak corresponding to the butterfly mode is lowered so that a dynamic characteristic of the actuator is improved.
However, in the actuator 90 having the above structure, since the two coils 92 and 94 are manufactured and coupled to the rear end portion of the actuator 90, manufacturing the actuator is difficult. Also, since the rear end portion of the actuator 90 becomes heavier due to the two coils 92 and 94, balancing the overall weight of the actuator is difficult. Furthermore, two input power sources, which can independently drive the two coils 92 and 94, are needed. In addition, it is difficult to perform servo control because the directions of the currents flowing through the respective coils 92 and 94 need to be changed by recognizing whether the actuator 90 is in the track following operation or in the seeking operation.